Proximity detectors for detecting ferrous articles and/or magnetic articles are known. The magnetic field associated with the ferrous article or magnet is detected by a magnetic field transducer, such as a Hall element or a magnetoresistance element, which provides a signal (i.e., a magnetic field signal) proportional to a detected magnetic field. In some arrangements, the magnetic field signal is an electrical signal.
One application for a proximity detector is to detect the approach and retreat of each tooth of a rotating ferrous or soft ferromagnetic gear. In some arrangements, a ring magnet having magnetic regions (permanent or hard magnetic material) with alternating polarity is coupled to the ferrous gear or is used by itself and the magnetic field sensor is responsive to approach and retreat of the magnetic regions of the ring magnet. The proximity detector processes the magnetic field signal to generate an output signal that changes state each time the magnetic field signal either reaches a peak (positive or negative peak) or crosses a threshold level. Therefore, the output signal, which has an edge rate or period, is indicative of a speed of rotation of the ferrous gear or of the ring magnet.
In one type of proximity detector, sometimes referred to as a peak-to-peak percentage detector (or threshold detector), a threshold level is equal to a percentage of the peak-to-peak magnetic field signal. One such peak-to-peak percentage detector is described in U.S. Pat. No. 5,917,320 entitled “Detection of Passing Magnetic Articles While Periodically Adapting Detection Threshold” and assigned to the assignee of the present invention.
Another type of proximity detector, sometimes referred to as a slope-activated detector or as a peak-referenced detector, is described in U.S. Pat. No. 6,091,239 entitled “Detection Of Passing Magnetic Articles With a Peak Referenced Threshold Detector,” which is assigned to the assignee of the present invention. Another such peak-referenced proximity detector is described in U.S. Pat. No. 6,693,419, entitled “Proximity Detector,” which is assigned to the assignee of the present invention and incorporated herein by reference. Another such peak-referenced proximity detector is described in U.S. Pat. No. 7,199,579, entitled “Proximity Detector,” which is assigned to the assignee of the present invention and incorporated herein by reference. In the peak-referenced proximity detector, the threshold signal differs from the positive and negative peaks (i.e., the peaks and valleys) of the magnetic field signal by a predetermined amount. Thus, in this type of proximity detector, the output signal changes state when the magnetic field signal comes away from a peak or valley by the predetermined amount.
Some proximity detectors use two types of detectors and switch between the two types, for example, as described in U.S. patent application Ser. No. 11/333,522, filed Jan. 13, 2006, entitled “Method and Apparatus for Magnetic Article Detection,” which is assigned to the assignee of the present invention and incorporated herein by reference.
It should be understood that, because the above-described peak-to-peak percentage detector and the above-described peak-referenced detector both have circuitry that can identify the positive and negative peaks of a magnetic field signal, the peak-to-peak percentage detector and the peak-referenced detector both include a peak detector circuit adapted to detect a positive peak and a negative peak of the magnetic field signal. Each, however, uses the detected peaks in different ways.
In order to accurately detect the positive and negative peaks of a magnetic field signal, the proximity detector is capable of tracking at least part of the magnetic field signal. To this end, typically, one or more digital-to-analog converters (DACs) can be used to generate a tracking signal, which tracks the magnetic field signal. For example, in the above-referenced U.S. Pat. Nos. 5,917,320 and 6,091,239, two DACs are used; one (PDAC) to detect the positive peaks of the magnetic field signal and the other (NDAC) to detect the negative peaks of the magnetic field signal.
Some types of proximity detectors perform one or more types of calibration, typically at a time near to start up or power up of the proximity detector. During one type of calibration, in particular for a peak-to-peak percentage detector (threshold detector), the above-described threshold level is determined. Immediately upon startup, the threshold may have a value that results in an improper output from the proximity detector.
In some applications, the ferrous or soft ferromagnetic gear or the ring magnet includes a “signature structure,” which is different than other parts of the ferrous or soft ferromagnetic gear or ring magnet. During operation, i.e., when the ferrous or soft ferromagnetic gear or ring magnet is rotating, the signature structure passes near to the magnetic field sensor, resulting in a “signature region” in the magnetic field signal generated by the magnetic field sensor. The signature region allows the proximity detector, or a processor that receives the output signal from the proximity detector, to identify an absolute angle of rotation of the ferrous or soft ferromagnetic gear or ring magnet. However, if the signature region is generated near to a time when the proximity detector is attempting a calibration (typically near to a start up time), the calibration of the threshold(s) can be degraded, resulting in an inaccurate output, or no output, from the proximity detector for at least a period of time.